Field emission displays are known in the art. They include an envelope structure having an evacuated interspace region between two display plates. Electrons travel across the interspace region from a cathode plate (also known as a cathode), which includes electron-emitting devices, to an anode plate (also known as an anode), which includes deposits of a light-emitting material, or "phosphors". Typically, the pressure within the evacuated interspace region between the cathode and anode plates is on the order of 10.sup.-6 Torr.
Spacers for field emission displays are also known in the art. The cathode plate and anode plate are thin in order to provide low display weight and reduce package thickness. If the display area is small, such as in a 2.54 cm diagonal display, and a typical sheet of glass having a thickness of about 0.1 cm is utilized for the plates, the display will not collapse or bow significantly. However, as the display area increases the thin plates are not sufficient to withstand the pressure differential in order to prevent collapse or bowing upon evacuation of the interspace region. For example, a screen having a 76.2 cm diagonal will have several tons of atmospheric force exerted upon it. As a result of this tremendous pressure, spacers play an essential role in large area, light-weight displays. Spacers are structures being incorporated between the anode and the cathode plate. The spacers, in conjunction with the thin, lightweight, plates, support the atmospheric pressure, allowing the display area to be increased with little or no increase in plate thickness.
Several schemes have been proposed to provide display spacers. These spacers and methods have several drawbacks. Methods for fabricating spacers which employ screen printing, stencil printing, or the use of glass balls suffer from the inability to provide a spacer having a sufficiently high aspect ratio (the ratio of spacer height to spacer thickness). Prior art methods which include aligning each individual spacer require repeated alignment steps and the adhesion of each spacer so that they will remain upright during subsequent packaging and sealing steps. These requirements are time consuming and add complexity.
Spacers for a field emission flat panel display must be invisible to the viewer. That is, they must be narrow enough to fit between the pixels of the display. Typically, the anode is coated with phosphor dots which are surrounded by a "black surround" material. This black surround material serves to increase the contrast ratio of the display and prevent the scattering of light between pixels. If the spacers are narrow enough to fit in the black surround area, then they will be invisible to the viewer. As an example, a 26.4 cm VGA display is easily designed to allow about 150 micrometers wide black surround stripes in one axis. Allowing for various tolerances, this means that spacers which are less than 100 micrometers wide would easily fit in the available space. It may be shown, for example, that glass rods having a diameters of 75 micrometers placed in a square pattern having a pitch of about 0.5 cm would satisfy both the invisibility and the structural strength requirements. The height of the spacer and, consequently, the aspect ratio is determined by the operating voltage. For a display designed to operate at 5,000 volts, a distance of about 1 millimeter is required between the anode and cathode plates of the display in order to prevent arc break down. The resulting aspect ratio is therefore about 10:1, which tends to make the spacers difficult to position and place accurately.
Accordingly, there exists a need for an apparatus and method for incorporating spacers into a field emission display which provides high aspect ratio spacers and ease of spacer placement and alignment.